Converter bodies have long been formed of a plurality of thin metal strips or layers wound about a central pin or about spaced "fixation" points. Such prior converter bodies, including prior catalytic converters, have utilized a support means both at the outer end of the individual layers as well as the inner end. The support means, at a minimum, has often been the housing for the converter body in combination with a central pin or post. In certain instances, the interior support means has been, at least in part, as a result of looping the thin metal layers about a fixed point or points whereby the inner ends of the thin metal layers have been supported by another thin metal layer. Most often, the thin metal strips or layers forming the multicellular honeycomb body have been brazed together intermediate the ends thereof whereby a rigid honeycomb monolith has been formed. In all instances, however, both the inner and the outer ends of the layers have been supported.
The support has been effected by soldering, welding, brazing, riveting, clamping, reverse wrapping or folding, or the like whereby the inner and outer ends, and usually the intermediate portion, of the layers or strips are fixedly secured to the support member. Varying degrees of success in passing tests prescribed by automobile manufacturers have been achieved.
It has now been found that the structure can be improved by allowing one end, e.g., the inner end, of the thin metal core elements to "float" in the fluid stream. Whereas it was previously thought that rigidity was essential to prevent failure in the "Hot Tests" (described below), it has been discovered that flexure or compliance of the thin metal core elements in response to thermal and fluid flow variations as well as mechanical vibration is a desirable attribute of the converter bodies. This discovery has given rise to what we term a "cantilever" converter body, i.e., one in which the thin metal core elements forming the core are secured at one end only, preferably at the outer end, in a spirally wound device. In such a construction, the individual thin metal core elements are "compliant," that is, they yield to stresses within the elastic limit of the thin metal.
This invention will be described in connection with embodiments especially adapted for use in the exhaust lines of various types of engines, e.g., internal combustion engines of the spark ignited or compression ignited types, stationary or mobile, or gas turbines. It will be understood that the converters of the present invention may be used to effect various chemical reactions, particularly those occurring in fluid streams, especially gas streams, and which reactions are catalyzed or uncatalyzed. A particular reaction is the oxidation of pollutant materials contained in exhaust streams from internal combusion engines.
Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since the early 1970's. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated 30 Oct. 1973. More recently, corrugated thin metal monoliths have been disclosed in U.S. Pat. No. 4,711,009 dated 8 Dec. 1987 to Cornelison et al which discloses a process for . . . making precoated corrugated thin metal strips in a continuous manner, and accordion folding them into predetermined shapes; U.S. Pat. Nos. 4,152,302 dated 1 May 1979, 4,273,681 dated 16 Jun. 1981, 4,282,186 dated 4 Aug. 1981, 4,381,590 dated 3 May 1983, 4,400,860 dated 30 Aug. 1983, 4,159,120 dated 28 May 1985. 4,521,947 dated 11 Jun. 1985, 4,647,435 dated 3 Mar. 1987, 4,665,051 dated 12 May 1987 all to Nonnenmann alone or with another and which disclose multicellular honeycomb monolithic converters with corrugated and flat thin metal strips having their contiguous surfaces brazed together; U.S. Pat. No. 5,070,694 dated 10 Dec. 1991 to Whittenberger which discloses spirally wound converters with corrugated strips and flat strips. International PCT Publication WO 90/12951 published 9 Apr. 1990 seeks to improve axial strength by form locking layers of insulated plates. Another reference which seeks to improve axial strength is U.S. Pat. No. 5,055,275 dated 8 Oct. 1991 to Kannainian et al. Reference may also be had to International PCT Publication No. 92/13626 filed 29 Jan. 1992. This application relates to a multicellular honeycomb converter body along an axis of which fluid can flow through a plurality of channels. The honeycomb has at least two discs in axially spaced relation to each other. According to the specification, there is at least one bar type support near the axis by which the discs are connected together and mutually supported. The invention is said to make possible design of the first disc for fast heating up through hot exhaust gas passing through, or applied electrical current. The honeycomb body serves as a support for catalyst in the exhaust system of an internal combustion engine. Another reference is German Patent Application 4,102,890 A1 filed 31 Jan. 1991 and published 6 Aug. 1992. This application discloses a spirally wound corrugated and flat strips combination wherein the flat strip contains slots and perforations and is electrically heatable. The flat strips include a bridge between leading and trailing portions. Groups of strips are separated by insulation means. Another reference is U.S. Pat. No. 5,102,743 dated 7 Apr. 1992. This patent discloses a honeycomb catalyst carrier body of round, oval, or elliptical cross-section including a jacket tube and a stack of at least partially structured sheet metal layers intertwined in different directions in the jacket tube. The stack has a given length and a given width. At least one of the sheet metal layers has a greater thickness over at least a part of one of the dimensions than others of the layers. Such at least one layer is formed of thicker metal or of a plurality of identically structured metal sheets in contiguous relation.
European Patent Application 0,322,566 filed 25 Nov. 1988 discloses a spirally wound honeycomb core formed of corrugated and flat thin metal strips. In this structure, the head ends in the center of the strips are grasped in the middle in order to coil the strips, or they can be grasped sequentially one after the other and coiled. The strips are weakened appropriately in the middle at the head ends of the strips by a pair of inwardly directed slots.
Reference may also be had to U.S. Pat. No. 4,832,998 dated 23 May 1989 to Cyron. This patent discloses an S-wound honeycomb converter body and a method of producing it, the body including a stack of structured metal sheets disposed in layers at least partially spaced apart from each other defining a multiplicity of channels through which gases can flow, the stack having ends looped in mutually opposite directions about at least two spaced fixation points, and a jacket tube surrounding the sheets and being formed of at least one segment, the sheets having the ends of the loops joined with the jacket tube. The devices have no central post. A method patent is U.S. Pat. No. 4,923,109 dated 9 May 1990 to Cyron, a division of the aforementioned Cyron patent, directed to the method of making the devices of the earlier U.S. patent.
Reference may also be had to U.S. Pat. No. 5,232,671 dated 3 Aug. 1993 to Brunson. This patent discloses an improved electrically conductive metal honeycomb body having a plurality of corrugated thin metal strips, which may be heater strips, extending in electrical parallel between otherwise electrically isolated connector plates. The corrugated thin metal strips have a flat central section. A first group of strips is gathered at their flat middle portions and bent around one of a pair of rigid central posts, and a second group of strips is gathered and bent in the opposite direction about the other of the posts. Insulation in the form of flexible woven ceramic fiber strips isolate the first and second groups from each other and from the central posts. The connector plates define a segmented retainer shell about the body. A battery is connected to the connector plates whereby current flows from one connector plate through the corrugated thin metal strips to the other connector plate and back to the battery.
As indicated above, a common problem with many of the prior devices has been their inability to survive severe automotive industry tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at a high temperature (between 800 and 1050 degrees C.; 1472 to 1922 degrees F., respectively) with exhaust gas from a gas burner or a running internal combustion engine simultaneously passing through the device. If the device telescopes, or displays separation or folding over of the leading or upstream edges of the foil leaves, or shows other mechanical deformation or breakage up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test.
The Hot Cycling Test is run with exhaust flowing at 800 to 1050 degrees C.; (1472 to 1922 degrees F.) and cycled to 120 to 200 degrees C. once every 10 to 20 minutes for 300 hours. Telescoping or separation of the leading edges of the thin metal foil strips, or mechanical deformation, cracking or breakage is considered a failure.
The Hot Shake Test and the Hot Cycling Test are hereinafter called "Hot Tests" and have proved very difficult to survive. The structures of the present invention will survive these Hot Tests. Other tests similar in nature and effect are known in the industry
In the following description, reference will be made to "ferritic" stainless steel. A suitable ferritic stainless steel is described in U.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen. A specific ferritic stainless steel alloy useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. A ferritic stainless steel is commercially available from Allegheny Ludlum Steel Co. under the trademark "Alfa IV."
Another stainless steel metal alloy especially useful herein is identified as Haynes 214 alloy which is commercially available. This alloy and other useful nickeliferous alloys are described in U.S. Pat. No. 4,671,931 dated 9 Jun. 1987 to Herchenroeder et al. These alloys are characterized by high resistance to oxidation and high temperatures. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon, and steel making impurities. Haynes 230 alloy, also useful herein has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, a trace amount of lanthanum, balance nickel.
The ferritic stainless steels, and the Haynes alloys 214 and 230, all of which are considered to be stainless steels, are examples of high temperature resistive, oxidation resistant (or corrosion resistant) metal alloys that are useful for use in making the thin metal core elements hereof, as well as the multicellular honeycomb converter bodies hereof. Suitable metal alloys must be able to withstand "high" temperatures, e.g., from 900 degrees C. to 1200 degrees C. (1652 degrees F. to 2012 degrees F.) over prolonged periods.
Other high temperature resistive, oxidation resistant metal alloys are known and may be used herein. For most applications, and particularly automotive applications, these alloys are used as "thin" metal, that is, having a thickness of from about 0.001" to about 0.005", and preferably from 0.0015" to about 0.0037". The housings, or jacket tubes, hereof are of stainless steel and have a thickness of from about 0.03" to about 0.08", e.g., 0.04" to 0.06".
Reference will also be made to ceramic fiber insulation. Details of a suitable ceramic fiber insulation will be found in U.S. Pat. No. 3,795,524 dated 5 Mar. 1974 to Sowman, and to U.S. Pat. No. 3,918,057 dated 28 Oct. 1975 to Hatch, for formaulations and manufacture of fibers useful in making tapes and mats useful herein. One such woven ceramic fiber material is currently available from 3-M under the registered Trademark "NEXTEL" 312 Woven Tape and is useful for insulation between the inner and outer housings hereof. Ceramic fiber mat is commercially available under the trademark "INTERAM" also available from 3-M. Between the thin metal core elements hereof, the only insulation is provided by refractory metal oxide coating on the surfaces of the thin metal strips, or layers.